The invention relates to the design and manufacture of integrated circuits, and more particularly, to techniques, systems, and methods for implementing metal-fill patterns on an integrated circuit.
In recent years, in IC manufacturing, chemical-mechanical polishing (CMP) has emerged as an important technique for planarizing dielectrics because of its effectiveness in reducing local step height and achieving a measure of global planarization not normally possible with spin-on and resist etch back techniques. However, CMP processes have been hampered by layout pattern dependent variation in the inter-level dielectric (ILD) thickness which can reduce yield and impact circuit performance. A common approach for reducing layout pattern dependent dielectric thickness variation is to change the layout pattern itself via the use of metal-fill patterning.
Metal-fill patterning is the process of filling large open areas on each metal layer with a metal pattern to compensate for pattern-driven variations. The manufacturer of the chip normally specifies a minimum and maximum range of metal that should be present at each portion of the die. If there is an insufficient amount of metal at a particular portion or “window” on the chip, then metal-fill is required to increase the proportion of metal in that portion or window. Otherwise, an insufficient amount of metal may cause bumps to exist in the finished chip. However, too much metal may cause dishing to occur. Therefore, the metal-fill process should not cause the die to exceed any specified maximum range of metal for the chip.
FIG. 1 shows a “fixed template” approach for performing metal-fill patterning, in which a template pattern is overlaid with the chip design, the results are tested with a separate analysis step, and then new fixed shapes are added or the starting point (offset) of the fixed shapes is shifted until the minimum density is met in every area.
To explain further, in this approach, a chip layout is divided into a set of delineated portions or windows. For each window, the metal features or “blockages” 103 are identified, as shown in window 102. If the proportion of metal in that window is below a specified minimum percentage, then metal-fill patterning is performed to increase the amount of metal. In many cases, the designer or manufacturer will specify a minimum distance around each blockage that should not contain the additional metal-fill. As shown in window 104, a fence 105 is established around each blockage 103 in the window to maintain this minimum distance around each blockage.
A fill template is selected to provide the metal-fill pattern. The fill template is a fixed pattern of uniform metal shapes, e.g., an array of 2 um×2 um shapes spaced apart by 2 um, as shown in the example fill template of window 106. Once a fill template has been selected, the fenced blockage window 104 is overlaid upon the fill template, resulting in the new chip layout as shown in window 108.
At this point, a determination is made whether the layout meets minimum and maximum metal requirements. In some cases, the selected metal-fill pattern may contain too much metal, causing the new layout to exceed maximum metal percentages as specified by the manufacturer. In other cases, the metal-fill pattern may contain too little metal, causing the new layout to fall beneath specified minimum metal percentages. In either case, a new metal-fill pattern must be selected and the overlaying process repeated.
In certain instances, the metal-fill pattern may be sufficient, but must be “shifted” to properly fit against the fenced blockage window. For example, it can be seen in portion 110 of window 108 that because of the uneven distances between blockages, the metal-fill pattern does not exactly fit within the spaces between the blockages. Thus, the fixed, regular pattern of the metal in the metal-fill causes portions 112 and 114 of the new layout in window 108 to contain less metal than other portions. This can be corrected by shifting the metal-fill pattern 106 against the fenced blockage window 104 until a more optimal metal percentage is achieved.
The process of re-selecting a new metal-fill pattern or shifting the metal-fill pattern and then re-performing the overlaying is iteratively repeated until the final layout satisfies the minimum and maximum metal percentage requirements for the chip. In effect, this fixed template approach may be seen as a trial and error approach in which multiple passes through the metal-fill selection/overlaying process is needed to achieve an acceptable metal percentage. This trial and error approach can be costly and inefficient, particularly if the iterative steps of the process must be manually performed. Moreover, as new chip designs become smaller, the required metal percentage requirements become even stricter, which may require even more passes through this process to achieve an acceptable metal percentage.
To overcome the disadvantages of these and other approaches, the present invention provides an improved method, system, and article of manufacture for implementing metal-fill for an integrated circuit. A disclosed embodiment calculates the best offset in each area to be filled and dynamically adjust shape widths and different shape lengths that best fill that area, in which only a single pass is needed to appropriately determine the metal-fill pattern. An embodiment also simultaneously optimizes across multiple metal-fill windows such that that the process will not add shapes in a window that would exceed the maximum density, while attempting to make all windows match the preferred density, and meeting the minimum density.
Also disclosed is a method, system, and article of manufacture for implementing metal-fill that is coupled to a tie-off connection. An embodiment that is disclosed comprises a method, system, and article of manufacture for implementing metal-fill having an elongated shape that corresponds to the length of whitespace. Also disclosed is the aspect of implementing metal-fill that matches the routing direction. Yet another disclosure is an implementation of a place & route tool incorporating an integrated metal-fill mechanism. Other and additional objects, features, and advantages of the invention are described in the detailed description, figures, and claims.